If OM is a Christian...confront him privately first and tell him to stop any and all contact with your wife and tell him it would be helpful that he not disclose your contacting him privately and confronting him privately. Be sure to slide in that if he refuses or further interferes with your marriage (as he is biblically forbidden to do) that you'll need to address this up his chain of command (with his pastor and/or officials at his church). He can't be on church missions or representing the church in any official capacity and behave sinfully. A private rebuke should work just fine and since he's already told her he wasn't interested....he SHOULD have nothing to do with her at your request because he wouldn't want to risk even the appearance of impropriety.
If he refuses ....you must follow through with seeking church discipline.
Same goes for your wife. Your counselor can't help you if the counselor doesn't know the truth, Sandi, above asked what your expectations were for telling the counselor and your answer should be "I only expect the truth and to deal with the truth - no more hiding in shame/fear we will address these problems honestly from this point forward - no more lies".
If OM is married, his wife should be made aware that your wife poses a threat to her marriage and family. Apologize on her behalf but the more you speak the truth the better.
I am not advocating exposure....I am merely discussing Christian obligations and responsibilities within the church. Your wife (and OM) have already developed too close a relationship and this involves a church leader...such leader and your church need to be aware of such in order to protect everyone involved. As uncomfortable as it sounds, private confrontation is the first step...then elders and church leaders need to be involved. Read up on how to confront a Christian regarding their sin and Church discipline for further insight.
The internet is 90% complaining and entitlement and I hate it because I deserve better!